Liquid compositions of (per)fluorinated ion exchange polymers are known for use in the manufacture, and possibly repair, of ion exchange membranes, for membrane coatings containing conductive and non-conductive particles, and for many other uses. While such compositions are sometimes referred to as solutions, the compositions are generally recognized as being dispersions (in other words, colloidal suspensions) of polymer particles.
Liquid compositions are typically prepared by dissolving/suspending in an appropriate aqueous or aqueous-alcoholic medium the (per)fluorinated ion exchange polymer. Methods useful for obtaining such liquid dispersions are notably taught in U.S. Pat. No. 4,433,082 (DUPONT DE NEMOURS) 21 Feb. 1984, GB 1286859 (DU PONT) 23 Aug. 1972, EP 1004615 A (AUSIMONT S.P.A.) 31 May 2000 and U.S. Pat. No. 6,150,426 (DUPONT DE NEMOURS) 21 Nov. 2000.
Generally, the above mentioned processes for dispersing (per)fluorinated ion exchange polymers involve harsh conditions, in particular high temperatures and thus require highly specialized equipment able to withstand corrosion from the aggressive acidic groups of the (per)fluorinated ion exchange polymer.
In such conditions, contamination of the dispersion with ions, in particular metal ions is generally unavoidable. Additional ionic impurities might derive from other manufacturing steps of the (per)fluorinated ion exchange polymer, like notably polymerization (e.g. residues from initiators) and/or hydrolysis steps.
Even low levels of ionic impurities (cations, in particular) contained in liquid compositions of (per)fluorinated ion exchange polymers are considered today as possibly negatively affecting the performance of membranes obtainable from the ion exchange polymers, in particular of membranes used in fuel cell applications. As a matter of fact, the presence of ions in any part of the fuel cell assembly might lead to the ‘neutralization’ of the acidic groups comprising mobile protons, which might in turn transform into ionically non-conductive counterparts. Furthermore, certain metal ions (e.g. Fe, Cr, Cu, Ti . . . ) have been recognized as acting as catalysts, in oxidative conditions like those of the fuel cell, for the formation of peroxide radicals that accelerate the degradation of the membrane.
In a nut-shell, very low levels of ions in the starting (per)fluorinated ion exchange polymer dispersions are believed to be associated with increased durability, performance, and quality of the final membrane: thus purification from ions, and especially from cations, has become part of good practice in the manufacture of liquid compositions of (per)fluorinated ion exchange polymers.
To this aim, US 2006014886 (3M INNOVATIVE PROPERTIES CO) 19 Jan. 2006 and US 2006014887 (3M INNOVATIVE PROPERTIES CO) 19 Jan. 2006 teach the use of cation exchange resins (e.g. polysulfonates or polysulfonic acids, polycarboxylates or polycarboxylic acids) for removing ions from ionic fluoropolymer dispersions.
Traditional ion exchange resins might release organic chemicals, in particular when exposed to strongly acidic conditions provided by the liquid composition of (per)fluorinated ion exchange polymers, so that risk of further contamination by organic compounds cannot be avoided. Also, ion exchange conditions with traditional resins are limited to room temperature, as these resins are often endowed with poor thermal stability.
US 20060199062 (ASAHI KASEI CHEMICAL CORP.) 7 Sep. 2006 discloses the possibility of removing the excess of alkali metals from a solution containing a perfluorocarbonsulfonic acid resin a polyazole-based compound and an alkali metal hydroxyde by means of either a strongly acidic cation exchange resin as well as by means of a dialysis treatment using a cation-exchange membrane. Such a dialysis treatment may not be sufficient to remove the alkali metal ions tightly bound to the perfluorosulfonic acid resin.